1. Field of the Invention
The present invention relates to an image forming apparatus and a control method for the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copier and a color printer, sheets and OHP sheets pass between an image carrying member and a transfer means and a toner image on the image carrying member is transferred onto the sheets. To execute efficient transfer at time of the transfer, a method for applying a transfer bias voltage to a transfer means is known. Depending on the difference in the method for applying the transfer bias voltage to the transfer means, the constant-voltage control method and constant-current control method are known.
The constant-voltage control method keeps the voltage to be applied to the transfer means as a transfer bias voltage at a constant voltage. In this case, the voltage applied to the transfer means is kept constant, so that when the resistance of the transfer means increases, the current flowing through the transfer means is reduced. Namely, even if the transfer can be executed appropriately in an environment at normal temperature and normal humidity, when the resistances of sheets, the transfer member, and image carrying member increase in an environment at low temperature and low humidity, a necessary transfer current cannot be obtained and defective transfer may be caused.
The constant-current control method keeps the transfer current flowing through the transfer means constant. By use of the constant-current control method, an appropriate transfer can be executed independently of the environment of temperature and humidity. However, even if the transfer is executed under the constant-current control, when the width of sheets is narrower than the width of the transfer means, the transfer means directly makes contact with the image carrying member, so that large currents flow on the contact part, thus a necessary current cannot be obtained on the sheets, and defective transfer is caused.
Therefore, as disclosed in Japanese Patent Application 2-264278, a control method in combination of the constant-voltage control with the constant-current control is proposed such that the transfer belt is in contact with the transfer means when there are no sheets P, and a constant current is applied to the transfer means, and the voltage generated in the transfer roller is assumed as V1, and when actually transferring on sheets, V1 is multiplied by a certain coefficient R, and the constant-voltage control is executed at a voltage of V2 (=RV1) higher than V1.
Further, as disclosed in Japanese Patent Application 2002-278307, the art for executing control for changing the transfer bias voltage depending on the intra-apparatus humidity, the sheet kind, and the count for printing when both sides of each sheet are printed or both sides are printed several times is proposed.
However, the difference between V1 and V2 in the aforementioned Japanese Patent Application 2-264278 is the difference in the transfer bias voltage generated by the resistances of sheets and toner and the value is changed widely depending on the environment of the sheet kind and intra-apparatus humidity. Therefore, when V2 is decided by multiplying V1 by a certain coefficient R, it may not be said always that only by multiplying V1 by a uniform constant R, an appropriate transfer bias voltage can be applied.
Further, in the aforementioned Japanese Patent Application 2002-278307, the necessary transfer bias voltage is taken into account depending on the intra-apparatus temperature and humidity and sheet kind, though the transfer bias voltage is a theoretical value decided on the basis of a predetermined numerical formula and table. Therefore, the actually necessary transfer bias voltage may vary with the use history of the image forming apparatus and the quality of sheets used by a user.